Engine systems are known which operate the engine with lean combustion, or a lean air/fuel ratio, to improve fuel economy. To accommodate lean burn conditions, emission control devices, such as nitrous oxide (NOx) traps, are used to adsorb nitrous oxide emissions produced during lean operation. Adsorbed nitrous oxide is periodically purged by operating the engine with rich combustion, or a rich air/fuel ratio.
During normal lean and rich operation, sulfur contained in the fuel can become trapped in the emission control device. This gradually degrades the emission device capacity for storing nitrous oxide, as well as the device efficiency. To counteract the sulfur effect, various sulfur decontamination methods are available.
One method for sulfur decontamination requires elevating the emission control device temperature to a predetermined value. Then, additional fuel is injected while the catalyst is at this elevated temperature to reduce the sulfur stored in the device. The temperature of the device is raised by operating some of the cylinders lean and some of the cylinders rich. When the lean and rich exhaust gases meet in the device, exothermic reactions takes place, thereby releasing heat to increase the device temperature. The lean and rich exhaust gases are kept at certain desired lean and rich air/fuel ratios to maintain the average air/fuel ratio of the mixed exhaust gases at a desired air/fuel ratio. The desired lean and rich air/fuel ratios are determined in table look-up fashion with various correction factors. An exhaust gas air/fuel ratio sensor is relied upon to correct the desired lean and rich air/fuel ratios for control errors in the correction factors. Such a method is described in U.S. Pat. No. 5,657,625.
The inventors herein have recognized a disadvantage with the above approach. In particular, the method described for maintaining the average mixed exhaust air/fuel ratio is cumbersome and overly complex. In addition the above method requires an additional exhaust air/fuel ratio sensor because the open loop methods are not robust. Extensive testing and development, along with excessive computer memory storage and cost are necessary to use the above method. In other words, a simple, straightforward, and accurate method is not shown for determining a desired lean air/fuel ratio for the lean cylinders and a desired rich air/fuel ratio for the rich cylinders such that a desired exhaust gas mixture air/fuel ratio is achieved.